1. Field of the Invention
The present invention relates to the fabrication of photomasks useful in the manufacture of integrated circuits.
2. Background of the Related Art
Photolithography techniques use light patterns and photoresist materials deposited on a substrate surface to develop precise patterns on the substrate surface prior to the etching process. In conventional photolithographic processes, a photoresist is applied on the layer to be etched, and the features to be etched in the layer, such as contacts, vias, or interconnects, are defined by exposing the photoresist to a pattern of light through a photolithographic photomask which corresponds to the desired configuration of features. A light source emitting ultraviolet (UV) light, for example, may be used to expose the photoresist to alter the composition of the photoresist. Generally, the exposed photoresist material is removed by a chemical process to expose the underlying substrate material. The exposed underlying substrate material is then etched to form the features in the substrate surface while the retained photoresist material remains as a protective coating for the unexposed underlying substrate material. Since photomasks are used repeatedly to create device patterns, quality control of photomask manufacturing is very important.
Photolithographic photomasks, or reticles, include binary (or conventional) photomasks and phase shift masks (PSM), which could be used in sub 0.13 μm technology. Binary (or conventional) masks typically include a substrate made of an optically transparent silicon based material, such as quartz (i.e., silicon dioxide, SiO2), having an opaque light-shielding layer of metal, such as chromium, on the surface of the substrate. Phase shift masks improve the resolution of the aerial image by phase shifting. The principle of phase shift mask is described in P. 230-234 of Plummer, Deal and Griffin, “Silicon VLSI Technology Fundamentals, Practice and Modeling”, 2000 by Prentice Hall, Inc. Phase shift masks could be either attenuated phase shift or alternate phase shift mask. An attenuated phase shift mask typically includes a substrate made of an optically transparent silicon based material, such as quartz, having a translucent layer of material, such as molybdenum silicide (MoSi) or molybdenum silicon oxynitride (MoSiON), on top. When the photolithographic light, e.g. at 248 nm wavelength, shines through the patterned mask surface covered by the translucent layer, the transmission (e.g. 6% at 248 nm wavelength) and the thickness of the translucent layer create a phase shift, e.g., 180°, compared to the photolithographic light that shines through the patterned mask surface not covered by the translucent layer. An alternate phase shift mask typically includes a substrate made of an optically transparent silicon based material, such as quartz, which is etched to a certain depth to create a phase shift with the un-etched transparent substrate when the photolithographic light shines through the patterned mask.
Photomasks allow light to pass therethrough in a precise pattern onto the substrate surface. The metal layer on the photomask substrate is patterned to correspond to the features to be transferred to the substrate. The patterns on the photomask could be 1×, 2× or 4× of patterns that will be patterned on the wafer substrate. Typically, a photolithographic stepper reduces the image of the photomask by 4× and prints the pattern on the photoresist covering the wafer surface. Conventional photomasks are fabricated by first depositing one to two thin layers of metal, which could either be opaque or translucent depending on the types of masks being formed, on a substrate comprising an optically transparent silicon based material, such as quartz, and depositing a photoresist layer on substrate. The photomask is then patterned using conventional laser or electron beam patterning equipment to define the critical dimensions in the photoresist. The top metal layer, typically opaque, is then etched to remove the metal material not protected by the patterned photoresist, thereby exposing the underlying silicon based material. For a binary mask, the photomask is formed after the metal etching step. While for attenuate and alternate phase shift masks, additional photoresist patterning and etching of transparent substrate or translucent metal layer are needed to form the photomask.
Since photomasks are used repeatedly to create device patterns, the accuracy and tight distribution of the critical dimensions, and the phase shift angle and its uniformity across the substrate are key requirements for binary and phase shift photomasks. Critical dimensions are defined here as the widths of features being measured and are affected by etching process. Overetching could enlarge the dimension, while underetching could result in wider dimension distribution or metal layer not completely etched. In the case of phase shift masks, the accuracy and tight distribution of the phase shift angle, typically 180°, are also key requirements. For attenuated phase shift mask, the phase shift angle is affected by the thickness and transparency of the translucent metal layer (e.g. MoSi), while for alternate phase shift mask, the phase angle is affected by the transparent material and its etch depth.
A conventional method of controlling critical dimensions for the photomasks comprises measuring the critical dimensions of the respective elements of the etched photomasks, statistically processing the results of such measurements, determining if the measurement passes the requirement, and adjusting the etch process performed on subsequent batches of the substrate. Unfortunately, this method does not compensate for substrate-to-substrate variations of the critical dimensions within a batch of substrates. Variables inherent to the etch process may broaden distribution for the critical dimensions. This means that the post-etch statistical distribution of the critical dimensions for etched structures may be broader than the pre-etch distribution of critical dimensions of the etched structure of the photomask. As such, some etched structures may have critical dimensions outside a pre-determined range of acceptable values.
A conventional method of monitoring phase shift angle for the photomasks comprises measuring the phase shift angles and the uniformity (or distribution) of the photomasks, statistically processing the results of such measurements, determining if the measurement passes the requirement, and adjusting the etch process performed on subsequent batches of the substrate. Variables inherent to the metal film deposition processes that affect metal film thickness and metal film transparency of an attenuated phase shift mask could make the phase shift angle measurement results of an attenuate phase shift photomask outside a pre-determined range of acceptable values. In addition, variables inherent to the etch process combined with the variables of the photomask lithography process may broaden the distribution of the etch depth, which affects phase shift angle and its uniformity for an alternate phase shift mask. As such, some etched structures may have phase shift angles outside a pre-determined range of acceptable values.
Therefore, there remains a need in the art for an improved method and apparatus for controlling the critical dimensions and monitoring phase shift angle and its uniformity of photomask in a semiconductor photomask processing system.